Precise regulation of therapeutic gene expression is a central approach to the treatment of many genetic disorders. Recent technologies aim to reverse dysregulated gene expression through the development and delivery of synthetic regulatory systems, for example, using engineered proteins that target responsive promoters to conditionally induce or silence therapeutic gene expression. These protein-DNA interaction systems are encoded in nucleic acid constructs and delivered to cells through traditional cell delivery methods, for examples, lentiviral, retroviral, and adenoviral vectors. It has importantly been demonstrated that first-generation therapeutic delivery systems are functional and clinically viable strategies capable of achieving long-term regulation in primates. Non-limiting examples of some first-generation therapeutic delivery systems include simple, zinc finger containing transcription factors to induce therapeutic gene expression.
However, there are fundamental limitations to certain families of synthetic regulatory proteins that prevent their widespread adoption in gene therapies. For example, certain classes of programmable DNA-targeting domains (Transcription Activator Like Effector (TALE) and CRISPR/dCas9) are derived from prokaryotic systems, rendering them likely to be immunogenic in a human therapy context. Additionally, these proteins are large and approach the packaging limits of traditional lentiviral delivery schemes, preventing ease of delivery and addition of other useful molecular components.